Adjusting Categories to the Regional Level

The IUCN criteria are designed to evaluate the risk of extinction at a global level. To adjust to the regional level requires further considerations as the regional status depends on the exchange taking place with populations in other regions. The threat category must be adjusted to a lower category if populations outside the region support the regional population. Mycelia do not migrate between regions, but their spores do, and if the species occur in an adjacent region, it is likely that there will be an influx of viable spores to the region. Such spore influx is likely to enhance the regional survival of small populations, especially for species occurring at the margin of their distribution area or close to an area where the species is more common. In the same way the category may be adjusted to a higher level if the regional population fitness is believed to depend on spore exchange with populations in adjacent regions, when these extra-marginal populations are known to be declining.

2.2 Fungi as Indicators of Habitat Quality

Saprotrophic fungi have been tested and applied as indicators of valuable habitats following two different approaches. In the first attempts to use fungi as indicators (e.g. Rald, 1985) the suggested indicator species were simply stated to indicate ''nature value'' or, circularly, sites valuable for the protection of the indicator species themselves. Others have mentioned ecological continuity or overall conservation value as indication goals (e.g. Karstrom, 1992; Bredesen et al., 1997). Most of the proposed indicator schemes have been suggested by experienced field mycologists, but few have been tested scientifically to determine if the indicator species really indicate what they are assumed to indicate. For this reason the relevance of some indicator schemes has been questioned. Most importantly, the use of selected polypores to indicate ecological continuity in boreal forests has been challenged (Norden and Appelqvist, 2001; Rolstad et al., 2002), due to lack of evidence, both theoretically and in practice. Partly based on this critique, more sophisticated approaches to test and use fungi as indicators of overall species richness have been developed. Some of those approaches have focussed solely on fungi, whereas others have investigated the potential of fungi as indicators of diversity in other organism groups—and vice versa.

Balmford et al. (2000) tested the usefulness of the so-called higher taxon approach in which the diversity at higher taxonomical levels (e.g. genera, families) is assumed to reflect diversity at lower taxonomic levels (typically species). Based on inventory data from 19 sites in England, a very high correlation between genus and species level richness was found, while richness at family and order level predicted species richness slightly less well. When data were used to select hypothetical reserve networks, richness at both the family and genus level was useful for indicating richness at the species level. Balmford et al. (2000) thus suggested inventories of genera rather than species to identify fungal diversity hotspots in a region, e.g. for inclusion in reserve networks. A related idea is the nested subset approach, which is based on the fact that species often form nested patterns across landscape habitat patches (Wright et al., 1998; Jonsson and Jonsell, 1999). Nested patterns occur if locally rare species tend to be confined to the most species rich localities/patches, while species poor localities/ patches, in contrast, only host locally common species. In the presence of nested patterns, subsets of species with intermediate or low numbers of occurrences accordingly may indicate overall species richness (Jonsson and Jonsell, 1999). The approach has been tested in relation to saproxylic fungi in boreal forests, but with inconsistent results. Jonsson and Jonsell (1999) failed to identify nested patterns among polypores in 10 selectively logged spruce forests, but nested patterns were identified in a similar study conducted in old growth forest patches in a natural wetland matrix (Berglund and Jonsson, 2003). Also S^tersdal et al. (2005) identified nested patterns among boreal wood-decay fungi, but patterns were highly inconsistent among three inventoried forest regions in Norway. Thus, indicator species, identified by nested subset analysis in one region, were typically non-indicative of species-rich stands in other regions.

Approaches focusing on the relation between fungal species diversity and diversity in other organism groups, typically analyse patterns of: (i) species richness coincidence and/or (ii) complementarities between groups. The first concept is based on the assumption that richness in one taxonomical or functional group may reflect richness in other groups, so that a well-known and easily surveyed group, e.g. birds or higher plants, can be used as a surrogate of diversity in less easily surveyed groups, e.g. fungi. Similarly, the complementarity approach assumes that complementarity or species turnover in one taxonomic group across a number of sites reflects complementarity or turnover in other groups. The typical aim is to facilitate the design of reserve networks protective also for less known species groups.

Most attempts to investigate species richness coincidence and complementarity patterns involving fungi have focussed on saproxylic species (Jonsson and Jonsell, 1999; Virolainen et al., 2000; Berglund and Jonsson, 2001; S^tersdal et al., 2004). In all cases the species richness coincidence among fungi and other inventoried groups of organisms was absent or low, but even complementarity patterns were vague. It thus seems that inventories based on other organism groups are poorly suited to detect the most species-rich sites for saproxylic fungi—and vice versa. This reflects different habitat requirements among groups. The diversity of saproxylic fungi is influenced mainly by the amounts and diversity of dead-wood habitats present, while diversity in other groups is influenced by other factors, e.g. soil productivity, forest climate or local stand continuity (e.g. Ohlson et al., 1997; Berglund and Jonsson, 2001). For other fungal groups Chiarucci et al. (2005) found that vascular plants were poorly suited to capture macrofungal species richness in hypothetical reserve networks in Italy. On the other hand, Schmit et al. (2005) found that tree species richness was a promising surrogate for fungal species richness at a large scale in a meta-analysis of 25 datasets from temperate and boreal forests from three different continents.

A general problem for all approaches focussing on species richness is that the most species-rich sites are not necessarily the most valuable for conservation (e.g. Gjerde et al., 2004; Heilmann-Clausen and Christensen, 2005). Threatened fungal communities are not necessarily species rich, and human disturbances threatening rare species may often increase rather than decrease overall diversity. If, for instance, half of an ancient beech forest hosting rare saproxylic polypores is cleared and planted with spruce, this is likely to introduce a large number of new species associated with this new host, while only a few very uncommon species associated with the ancient beech forest will disappear in the short term. Similarly eutrophication is likely to increase species richness but certainly not ''naturalness'' in naturally nutrient-poor habitat types, e.g. sphagnum bogs. There is accordingly a need for novel approaches bridging rigorous scientific testing with the early intensions of indicators as species pointing to threatened and conservation demanding fungal habitats.

One way forward could be to focus directly on red-listed species. Monitoring of all red-listed species is not practicable, because of the high number of species involved, e.g. 632 in Sweden alone (Gardenfors, 2005), and their rarity and spatial patchiness (Straatsma and Krisai-Greilhuber, 2003). It would be very useful to identify easily identifiable and not too rare indicators of sites with high numbers of red-listed species, e.g. by applying nested subset approaches with red-listed rather than total species richness in focus. Another possibility is to investigate further the relations between habitat qualities and the occurrence of rare or red-listed species, e.g. relations between habitat continuity at various geographical scales and the presence of certain fungal communities. Such analyses may or may not show that easily monitorable habitat parameters are more efficient predictors of the fungal conservation value than a suite of indicator species.

In summary, the use of fungal indicator species is rarely scientifically proved, and the most important aspect of fungal indicator species may be that they are attractive to interested fungal amateurs. Thus, they may be very powerful in increasing the awareness and knowledge of valuable fungal habitats, which would be difficult to achieve if the same group of people were encouraged to investigate habitat parameters, such as soil productivity, abundance of veteran trees or local habitat continuity through interviews with land owners or costly historical studies. This has, for instance, been the case with Hygrocybe species and other saprotrophs in grasslands, as discussed below.

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